Mill should add the multi-release jar file attribute to the created uber jar file if one of the input jar files is a multi-release jar.

[Flowdalic]

Mill should add the multi-release jar file attribute to the created uber jar file if one of the input jar files is a multi-release jar.

Right now, mill users have to manually add

  def manifest = super.manifest().add(
    "Multi-Release" -> "true"
  )

to their build.sc if one of the input jar files is a multi-release jar file and if they want the JVM to use the most suitable classes depending on the JVM version. Otherwise, this may leads to superfluous warnings because of code paths for older JREs being used.

^{This is (probably) the right fix, instead of #2724}